| Application of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has led to big breakthrough of analytical techniques for metal stable isotopes, resulting in rapid progresses in non-traditional stable isotope geochemistry. As a new geological tracer, Mg isotopes have been widely applied in studies of almost all important disciplines of geochemistry. High precision Mg isotope data measured by MC-ICP-MS are now available with precision about0.05%o amu-1(2SD) or better. Because mass bias caused by chemical procedure and instrument can easily induce significant analytical error, it is still a challenge to obtain accurate Mg isotope data for natural samples.In this paper, we systematically review the development of analytical technique of Mg isotopes, with a detailed description of a series of important techniques used in the measurement process, including calibration of instrumental mass-bias, chemical purification process, matrix effect, and pitfalls for high precision isotope analyses. We compare standard data from different labs and establish a guideline for Mg isotope analysis procedure. Additionally, we briefly discuss the behaviors of Mg isotopes during geological processes including equilibrium and kinetic Mg isotope fractionations, such as magma differentiation, chemical and thermal diffusion, and continental weathering. Finally, we propose some future prospects for Mg isotope geochemistry in both high and low temperature geological processes.We present a method for precise measurement of Mg isotope ratios for low-Mg rock samples (where MgO<1wt.%) by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The efficiency of Mg purification is significantly improved by using a newly calibrated HNO3+HF step to remove undesired matrix elements (such as Ti, Al, Fe, and K) in low-Mg samples. We also establish that increasing the amount of Mg loaded to the chromatographic column minimized blank effects of organics leached from cation resin. All parameters that could affect the accuracy and precision of Mg isotope analyses were rigorously examined by two independent laboratories in Beijing and Hefei. The δ26Mg of mono-elemental Mg standard CAM-1measured in the two laboratories were-2.597±0.042%o (2a, n=49) and-2.598±0.039%o (2a, n=79), respectively; in house standard IGGMgl were-1.742±0.041%o (2a, n=53) and-1.749■0.049%o (2a, n= 72), respectively. The average δ26Mg over ten months of two synthetic standards, made by doping IGGMg1and IGGMg2with matrix elements, agrees well with their recommended values, within error. The robustness of our method was further assessed by replicated analyses of sixteen rock standards with MgO contents from0.28wt.%to49.4wt.%. The δ26Mg of USGS rhyolite standards RGM-1and RGM-2are-0.188±0.031%o (2σ, n=35) and-0.182±0.041%o (2σ, n=72), respectively; granite standard GA is-0.165±0.038%o (2a, n=57), G-2is-0.129±0.045%o (2σ, n=34), GS-N is-0.204±0.059%(2σ, n=33), GSP-2is0.042±0.020%o (2σ, n=15), and GSR-1is-0.234±0.016%o (2σ, n=17). Based on repeated analyses of standards, the long-term external precision of our method is better than±0.05‰for δ26Mg. This precision allows us to distinguish the fractionation of Mg isotopes in low-Mg granites and rhyolites as well as that between mantle minerals.Studies of the Mg isotope composition of the Earth are important for understanding its geochemistry and may shed light on the accretion history of the Earth. However, there is uncertainty in the magnitude of Mg isotope fractionation at mantle temperatures and whether the Earth has a chondritic Mg isotope composition or not. Knowledge about the composition of the mantle is largely derived from studies of oceanic basalts, komatiite, mantle-derived minerals like olivine and pyroxene, peridotite xenoliths. Fresh mantle xenoliths captured by alkaline basalts, kimberlites and lamproites directly sample the sub-continental lithospheric mantle, and hence can provide constraints on Mg isotope composition of the upper mantle. We present high-precision measurements of Mg and Fe isotope compositions of coexisting olivine, clinopyroxene (cpx), orthopyroxene (opx), garnet (grt), and phlphite (ph1) from eight garnet lherzolite and harzburgites from the Kaapvaal Craton, in order to further constrain the Mg isotope composition of the terrestrial mantle and investigate the inter-mineral Mg and Fe isotope fractionation in high-temperature igneous minerals. Based on repeated analyses of standards and natural samples, the long-term external precision of Mg and Fe method are better than0.05‰for δ26Mg and δ56Fe.δ26Mg value in whole rock varies slightly from-0.236%o to-0.210%o with an average of-0.221±0.034%o (2SD, n=48). We also report the Mg isotope compositions of8garnet lherzolite from the Siberia Craton, with δ26Mg value of-0.227±0.036%o (2SD, n=23), consistent with that from the Kaapvaal Craton. These results are indistinguishable from the isotope composition of the fertile upper mantle represented by spinel peridotites, likely suggesting homogeneous Mg isotope compositions in the Bulk Silicate Earth. Mg isotope compositions of Cpx, Opx and Ph1are variable and slightly heavier than coexisting01(-0.236±0.042‰,2SD, n=32), while grt has lighter Mg isotope compositions (-0.718±0.037‰～-0.517±0.047%o) than01. There are significant inter-mineral Mg isotope fractionations due to different coordination environment of Mg in minerals. Specifically, pyrope, where Mg is in eight-fold coordination, is more enriched in light Mg isotopes than olivine and pyroxene where Mg is in six-fold coordination. However, non-equilibrium processes exist for the Fe isotope compositions of mantle minerals between pyrope and olivine/pyroxene. The slightly heavy δ26Mg and significantly light δ56Fe of sample JGF-98/6relative to other normal mantle xenoliths can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion between infiltration melt and peridotite wall-rock. The Mg-Fe isotope study of garnet peridotites from ancient craton can further deepen our knowledge about the composition of the mantle up to180km. |
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